Joint replacement spacers

ABSTRACT

Devices and methods are disclosed for joint replacement.

SUMMARY

Devices and methods are disclosed for joint replacement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a joint.

FIG. 2 schematically shows a spacer for hemi-joint replacement.

FIG. 3 schematically shows an exploded view of a spacer for hemi-jointreplacement positioned in a joint.

FIG. 4 schematically shows a cutting implement for shaping a bone toreceive a spacer for hemi-joint replacement.

FIG. 5 schematically shows a cutting implement for shaping a bone toreceive a spacer for hemi-joint replacement.

FIG. 6 schematically shows an anterior view of a spacer for hemi-jointreplacement in the proximal phalangeal joint.

FIG. 7 schematically shows a medial view of a spacer for hemi-jointreplacement in the proximal phalangeal joint.

FIG. 8 schematically shows a cutting bit and guide for shaping a bone toreceive a spacer for hemi-joint replacement.

FIG. 9 schematically shows two views and a cross-section of a spacer forhemi-joint replacement in the talonavicular joint.

FIG. 10 schematically shows a bit for shaping a bone to receive a spacerfor hemi-joint replacement.

FIGS. 11, 12 and 13 schematically show a bit, bushing and mount forshaping a bone to receive a spacer for hemi-joint replacement.

DETAILED DESCRIPTION

A spacer for joint replacement can be used to replace a portion of bonein a joint. Such spacers and methods for using such spacers aredescribed in U.S. Pat. No. 8,303,664, issued Nov. 6, 2012, which ishereby incorporated herein by reference in its entirety. The joint isoriginally formed by two bones, each having an articulating surface,either of which may have some, all or none of the related cartilageremaining. The joint schematically shown in FIG. 1 includes a first bone101 with an articular surface 102 meeting a second bone 103 with anarticular surface 104. The cartilage is not drawn separately forsimplicity, but is meant to be included in each schematically drawnbone, to the extent that cartilage remains. A patient may need to have aportion of the first bone replaced, while the second bone remainsrelatively healthy with a functional articular surface. In that case, afull joint replacement, in which portions of both bones are removed, isundesirable. Instead, a hemi-joint replacement spacer can be used toreplace a portion of the first bone, while leaving the second boneentirely intact. It is desirable to leave as much healthy boneundisturbed as possible. The joint replacement spacer can be designed tointeract with the natural articular surface of the second bone, or elseto articulate with a prepared surface of the second bone.

A wide variety of combinations of curved elements could be used togenerate any particular cut surface so as to be complementary to aspacer. A cut surface can be formed with multiple radii of curvaturegenerated by two or more generators, or “surface-generating curves.”Typically the surface-generating curves will be planar. In someembodiments, a cutting bit will include two different radii of curvatureon the cutting surface and a third radius of curvature will beintroduced by translating or rotation the bit through another curve,located on either a collar or on a cutting guide. In some embodiments acutting burr will generate a single radius of curvature while othercurves and radii of curvature are generated by the collar, or by theguide, or by both the collar and the guide. One of skill in the art willrecognize that many different combinations of generators are possible,and any system that generates the necessary three (or four, or more)radii of curvature would suffice. Some radii of curvature will beinfinite, meaning that at least a portion of some generating curves maybe flat.

Several examples are shown schematically in the figures and aredescribed below.

FIG. 2 schematically shows a hemi-joint replacement spacer 201. Thespacer 201 has an articulating surface 202 for (a) replacing thearticular surface of a first bone, and (b) articulating with a secondbone. In reality, the articulating surface 202 would probably not besubstantially flat as shown, but would rather be curved to mimic theremoved articular portion of the first bone. The spacer 201 also has asecondary surface 203, at least part of which, or in some cases all ofwhich, is a stabilizing surface sized and shaped to fit against a cutsurface of the first bone. The cut surface can be prepared so as to begenerally complementary to the stabilizing surface. In contrast to jointimplants that are affixed to the bone by an interference fit, a surfaceto allow ingrowth, cement, screws, or the like, when the spacer 201 isin place on the cut surface of the first bone, the spacer is allowed asmall amount of movement relative to the bone. It is the interaction ofthe stabilizing surface and the cut surface of the first bone thatgenerally stabilizes the spacer relative to the first bone withoutcompletely immobilizing the spacer relative to the first bone.

FIG. 2 shows that the spacer can have a first axis 204 and a second axis205. The two axes may be perpendicular to one another, as shown in FIG.2, or they may be non-perpendicular. One or both axes may be curved. Thespacer 201 shown in FIG. 2 has a specific shape in each cross-sectionperpendicular to the axes. In each cross-section, the spacer 201 definesa curve 206, 207. The first curve 206 has first and second portions 208,209 that have non-equal radii of curvature shown as R₁ and R₃respectively. The second curve 207 has third and fourth portions 210,211 that have non-equal radii of curvature shown as R₁ and R₂respectively. Although FIG. 2 shows the first and third portions havingthe same radius of curvature, R₁, in some embodiments the first andthird portions may have non-equal curvatures. In the picturedembodiment, the radius of curvature of the first and third portions, R₁,is equal to the radius of a circular bone cutting tool, described inmore detail below. FIG. 2 also shows that the second curve 207 includesa fifth portion 212. In this case, the fifth portion 212 is flat, i.e.it has zero curvature, or an infinite radius of curvature, but inpractice a fifth portion could have any radius of curvature. In thiscase, the spacer also includes a bump 213, which is an optional featureof the stabilizing surface. When using a spacer 201 that includes a bump213, after forming the cut surface with the complementary radii ofcurvature, a cavity will also need to be formed complementary to thebump 213.

The spacer shown in FIG. 2 has a generally convex stabilizing surface,and is designed to be seated on a complementary, concave, preparedsurface of the first bone. Alternatively, the stabilizing surface can begenerally concave, designed to be seated on a complementary, convex,prepared surface of the first bone. Other possible shapes include thecases where either the second portion or the fourth portion is flat withno curvature (i.e., infinite radius of curvature). Also the stabilizingsurface could be saddle-shaped, so that the second portion is concavewhile the fourth portion is convex, or vice versa. In some saddle-shapedembodiments the radii of curvature of the second and fourth portions maybe equal in magnitude, but opposite in sign. In that case the two radiiof curvature of the second and fourth portions are considered to be notequal.

As shown in FIGS. 2 and 3, the perimeter of the spacer is generallyelliptical. Without altering the relationship of the various portions ofthe first and second curves, the perimeter could have essentially anyshape in the x-y plane shown in FIG. 3, such as a circle, oval,trapezoid, parallelogram, kite, triangle, or any other shape that wouldhelp the space to mimic the anatomy of the replaced portion of bone.

FIG. 3 schematically shows the spacer 301 above a bone 302 in anexploded view. The convex stabilizing surface 303 of the spacer 301 isvisible along with the concave prepared surface 304 of the bone 302. Thespacer is shown oriented in space by a three-dimensional Cartesiancoordinate system with axes x, y and z, and with arrows indicatingrotation about each of the three axes. Rotation about the x axis is“roll,” rotation about y axis is “pitch,” and rotation about the z axisis “yaw.” In this particular embodiment, the horizontal x and y axes arethe first and second axes of spacer.

One property of the spacer 301 shown in FIG. 3 is that, when thestabilizing surface 303 is seated against the prepared surface 304 ofthe bone 302, the spacer is substantially prevented from yawing, i.e.,rotating about the z axis. Because the first and third portions havedifferent radii of curvature, the spacer 301 cannot yaw and remain fullyseated against the bone 302; any rotation of the spacer 301 will tend tocause it to ride up off of the prepared surface 304. If the second andfourth portions had equal radii of curvature, the spacer might be ableto yaw. But because the second and fourth portions are not equallycurved, the aspect of the cut surface that has been shaped to match thesecond portion cannot also match the fourth portion. Any yawing willtherefore tend to cause the spacer 301 to ride up and off of the cutsurface 304. To the extent that the reapproximated joint holds thestabilizing surface 303 down onto the cut surface 304, the spacer 301will be inhibited from yawing. Because the cut surface 304 will never beperfectly complementary, it may be possible for the spacer 301 to yawslightly, but generally, the mating surfaces will inhibit the spacer 301from riding up and off the surface 304. Similarly, the fact that thefirst and second radii of curvature are not equal will tend to preventthe spacer 301 from pitching, or rotating about the y axis. The factthat the third and fourth radii of curvature are not equal will tend toprevent the spacer 301 from rolling, or rotating about the x axis.

FIG. 4 schematically shows a cutting implement, or bit, or burr, 401useful for removing portions of bone having a defined radius ofcurvature. The bit is configured to be attached to a powered rotationaldriver, for example an air driven drill or auger, or any other source orrotational motion. The bit 401 has a rotating cutting blade. One portion402 of the cutting blade defines a particular first radius of curvatureshown as R₁. A second portion 403 of the cutting blade defines a radiusof curvature shown as R₃. The implement 401 also has a collar 404. Inthis embodiment the collar can also have radius of curvature R₃,although in other similar embodiments, the collar could simply be flat.The collar is designed to ride on a guide 405. This particular guide isgenerally oval shaped, and is intended to be secured to the first bone,positioned directly above the portion of the first bone to be removed.The edge 406 of the guide defines a radius of curvature R₂. To form thecut surface, the bit 401 is plunged downward until the collar 404contacts the curved edge 406. The rotating bit is then swept along theguide 404 so as to form the cut surface on the bone. A cross-section ofthe resulting surface parallel to the long axis of the guide will haveradii of curvature R₁ near the edge and R₂ in the middle. Across-section of the resulting surface parallel to the short axis of theguide will have radii of curvature and R₁ near the edge and R₃ in themiddle. Thus, the resulting cut surface will be complementary to aspacer 201 like the one shown in FIG. 2, except for the bump 213. Asshown in FIG. 4, the cutting portion 402 of the bit 401 at its widestpoint has a width marked W which is roughly equal to, or slightlysmaller than, the width of the opening in the guide 405, also marked W.In this embodiment, the bit 401 would be plunged downward to meet theguide 405 and then swept back and forth in only one direction.

As an alternative, the bit could be formed as a hemispherical cuttingsurface with radius of curvature R₁ at all points on the cuttingsurface. In that case, a different part would need to generate the R₃radius of curvature. One possibility would be to make the bit have awidth significantly smaller than the inner dimension of the guide 405.Then the user would be free to sweep the bit in two dimensions over theguide. The collar 404 can then be formed with radius of curvature R₃ togenerate that radius of curvature as the bit is swept along thedirection of the short axis of the guide 405.

As shown in FIG. 4, the bit 401 used in combination with the guide 405would result in a concave prepared surface. As an alternative, the bit401 and guide 405 could be designed to result in a prepared surface thatis convex. The radius of curvature of the collar 404 could be invertedso that the collar is concave with radius of curvature R₃. And likewiseradius of curvature of the curved edge 406 of the guide 405 could beinverted so as to have a convex radius of curvature R₂. In that case,with a concave collar and a convex guide, the resulting prepared surfaceof the bone would be convex. As still another alternative, both thecollar and the guide could be convex, resulting in a saddle-shapedprepared surface. As still another alternative, both the collar and theguide could be concave, resulting in a different saddle-shape for theprepared surface.

FIGS. 2-4 can also be thought of as an example of the more generalstatement of the properties of the stabilizing surface and complementarycut surface as defined by planar generating curves. In the case wherethe cutting surface 402, 403 of the bit 401 has two radii of curvatureR₁ and R₃ and the collar 404 is flat, the cutting surface defines oneplanar generating curve having, in this case, two radii of curvature.That generating curve is combined with the planar generating curvedefined by the curved edge of the guide 406. By combining (1) the curvedefined by the cutting surface 402, 403 of the bit, and (2) the curvededge of the guide 406, a surface is created. The surface has twogeometrically independent aspects, namely the first and second axes 204,205. Along these geometrically independent aspects, that is, incross-sections perpendicular to the two axes, the surface has two radiiof curvature. In one geometrical aspect, the radius of curvature variesfrom R₁ to R₂ and back to R₁ so that the radius of curvature isnon-constant. In the other aspect, the radius of curvature varies fromR₁ to R₃ and back to R₁ so that the radius of curvature is againnon-constant in a second geometrical aspect.

“Geometrically independent aspects” is used herein to denotenon-congruent axes such that every point on the surface has a uniquecoordinate with respect to the axes. Examples of geometricallyindependent aspects include perpendicular linear axes, as shown in FIG.2, non-perpendicular linear axes, and curvilinear coordinate systemssuch as cylindrical systems in which one aspect is radial and another isazimuthal, toroidal systems in which radial distance is measured from acircle of fixed radius, and azimuth is measured about the center of thefixed circle, toroidal systems in which the fixed radius of the toroidvaries with angle for example elliptical, hyperbolic, or paraboliccoordinate systems, etc. In the case of linear axes, a “cross-section”perpendicular to the axis is easy to visualize since it is entirely in asingle plane. In the case of a coordinate system with an angular axis,at least one cross-section may be harder to visualize, since it may liein a more complicated surface, such as a circular, elliptical, parabolicor hyperbolic cylinder, rather than a simple plane as in the case oflinear axes. Nonetheless, the intersection of the stabilizing surfacewith such a geometrical surface defines a type of cross-section and iswell-defined. In the case of curvilinear coordinates, the existence of acurve in the coordinate system may constitutes one of the generatingcurves of the surface.

FIG. 5 schematically shows a similar bit 501 with a first radius ofcurvature R₁ on the “edge-cutting” portion 502 of the bit 501, and adifferent radius R₃ on the “plunge-cutting” portion 503 of the bit. Thecollar 504 also has radius of curvature R₃. This bit 501 could be usedeither with a guide whose width matched the width of the widest part ofthe edge-cutting portion 502, in which case the plunge-cutting portionwould entirely generate the aspect of the cut surface having radius ofcurvature R₃ while the bit 501 was swept in only one direction in theguide. Or the bit 501 could be used with a wider guide that allowed thebit 501 to be swept in a second direction, so that both the collar 504and the plunge-cutting portion 503 define the aspect of the cut surfacehaving radius of curvature R₃. In the case where the width of the guidematches the width of the bit, the cutting portion of the bit defines agenerating curve with two radii of curvature, while the guide definesanother generating curve with a single radius of curvature. The twoplanar curves combine in the same way described with respect to FIGS.2-4.

FIG. 6 schematically shows one embodiment of a particular spacer 601.This embodiment is particularly useful in a foot at themetatarsal-phalangeal joint. The articulating surface 602 of the spacer601 articulates with the metatarsal head and the spacer replaces theproximal articular surface of the proximal phalanx. As implanted in thefoot, FIG. 6 presents what would be an anterior view of the spacer 601.The opposite or inferior side of the spacer 601 has an outer edgeportion 603 with a first radius of curvature R₁. The central portion 604of the inferior side has a different radius of curvature R₃ in thiscross-section. FIG. 7 schematically shows the same spacer 601 but in alateral view. The articular surface 602 is still visible. The edgeportion 603 is also still visible showing radius of curvature R₁. Thecentral portion 604 is also still visible, but from the lateraldirection, the other radius of curvature, R₂, is now visible. Inferioror superior views of the spacer 601 would show a generally trapezoidalshape with rounded corners.

The particular embodiment shown in FIGS. 6 and 7 includes an optionalstabilizing bump 605. In addition to the stabilizing surface, which iscomplementary to the prepared bone surface incorporating multiple radiiof curvature as explained above, the bump is complementary to a cavityformed on the bone surface. The bump fits inside, but does not entirelyfill, the cavity. In this way the bump can allow for some limited motionof the spacer on the prepared surface, to the extent that the bump canmove within the cavity. This is explained in more detail in U.S. Pat.No. 8,303,664, issued Nov. 6, 2012, which is incorporated herein byreference in its entirety.

FIG. 8 shows a bit 801 for cutting and a guide 802 for cutting a surfacein a talonavicular joint. Unlike the bits in FIGS. 4 and 5, bit 801cannot plunge cut. The cutting surface 803 can only cut perpendicular tothe rotational axis of the bit 801. The cutting surface 803 can have endportions with radii of curvature R₁ and a central portion with adifferent radius of curvature R₂. The bit 801 includes an end collar 804and a pivot collar 805. When the guide is secured to the bone and thebit 801 is seated on the guide 804, the pivot collar 805 sits in pivotgroove 806 while the end collar 804 rides in the shaping groove 807. Theshaping groove 807 can have end sections 808 with a radius of curvatureR₃ while the central portion 809 can have a different radius ofcurvature R₄. The surface of the bone is prepared by swinging the bit801 side to side, allowing the pivot collar 805 to remain essentiallystationary in the pivot groove 806. The bit 801 moves vertically due toend collar 804 riding over the shaping groove 807. In some cases, R₁equals R₃ so that the resulting prepared surface has a single radius ofcurvature around its perimeter. Because the bit 801 pivots, itnecessarily sweeps out an arc. This is in contrast to the bits shown inFIGS. 4 and 5, which can be used with entirely rectilinear motion ifdesired. The resulting prepared surface of the bone will have differingradii of curvature in the cross-sections perpendicular to an axis, thesame as described in reference to FIGS. 2, 6 and 7. But in this case, atleast one axis is an arc, rather than a line. Such an arc is shown as adashed arc in FIG. 8. Additionally, the because at least one axis isarcuate rather than a straight line, the cut surface and correspondingsurface of the complementary implant will be stable against rollingabout the arcuate axis even if cross sections perpendicular to thearcuate axis are curved with only a single radius of curvature.

In the case of FIG. 8, the cutting surface 803 defines one planargenerating curve. Another generating curve is the arc through which thebit 801 and its cutting surface 803 is pivoted defines anothergenerating curve. The resulting cut shape, in the simplest case wherethe bit pivots in a planar, circular arc, has an outline similar to akidney bean, or cashew nut. In a radial cross-section, that is in avertical plane that contains the axis of rotation of the bit, thesurface has exactly the shape of the cutting surface 803, perhaps havingtwo different radii of curvature. In an independent geometrical aspect,that is, in a cross-section lying in a vertical circular cylinderdefined by the arc through which the bit 801 is pivoted, the curvedefined by the surface is flat bottomed (an infinite radius ofcurvature) with rounded edges whose curvature is defined by the radiusof the bit 801 (a finite, non-zero radius of curvature). In this way,the resulting surface can have non-constant radius of curvature in eachof two geometrically independent aspects.

An additional level of complexity is added if further changes incurvature are added to the various aspects of the surface. As notedabove, the shaping groove 807 can have multiple portions with differentcurvature in a vertical plane so that the collar 804 rides up and downas the bit 801 is pivoted. Additionally the shaping groove 807 can haveradial curvature, so that as a user pivots the bit 801, in order to keepthe collar 801 mated to the shaping groove 807, the bit 801 must movealong its axis of symmetry through the pivot groove 806. The resultingshapes can have a wide variety of curvilinear independent geometricalaspects.

FIG. 9 shows an example of an implant 901 that could fit on the surfaceprepared by the bit 801 and guide 802 shown in FIG. 8. FIG. 9 shows asuperior view of the implant looking down on to the articulatingsurface, a lateral elevation showing the perimeter portion of thestabilizing surface with radius of curvature R₁ and a central regionwith a convex radius of curvature R₃, and a cross-section in the coronalplane showing a concave radius of curvature R₂. In this example, thespacer has a saddle shaped articulating surface 902. The saddle shapecan be seen in that the medial view of the spacer shows the articulatingsurface 902 as concave, while the coronal plane cross-section, whichgives an anterior view, shows the articulating surface 902 as convex.Likewise, the stabilizing surface 903 is at least partially saddleshaped. The saddle shape can be seen in that the medial view of thespacer shows the stabilizing surface 903 as convex while the anteriorview coronal-plane cross-section shows the stabilizing surface 903 asconcave.

FIG. 10 shows another bit 1001 for use with a pivot type of guide,similar to the guide 802 shown in FIG. 8. This bit 1001 has a moreexaggerated difference between the different portions of the cuttingsurface. Perimeter portions 1002, 1003 are shown having radius ofcurvature R₁ while the central portion 1004 of the cutting surface hasradius of curvature R₂. Like the other bits shown herein, the bit 1001is fixedly attached to a drive shaft 1005. In this example, the driveshaft protrudes 1006 from the far side of the cutting surface to allowfor the attachment of a collar at the far end if desired.

In many of the embodiments described herein, for example the embodimentshown in FIG. 5, a rotating drive shaft turns the bit while a collar orbushing of some kind surrounds the drive shaft. In some cases the collarrotates with the shaft, so that the rotating collar rides on a guide. Inthat case, the collar will need to be rotationally symmetric about theaxis defined by the drive shaft. In other cases, the collar or bushingwill not rotate with the drive shaft, but rather the drive shaft willrotate within the collar or bushing. In that case, the collar or bushingneed not be rotationally symmetric, or symmetric at all.

FIGS. 11, 12 and 13 schematically show a different combination of a bit1101, a bushing 1102 and a mount 1103 to be used together to form asaddle shaped prepared surface on the bone. The bit 1101 is aside-cutting bit like those of FIGS. 8 and 10. The bit 1101 includesperimeter cutting portions 1104, 1105 defining a radius of curvature R₁and a central cutting portion 1106 defining a radius of curvature R₂.The bit also includes a drive shaft 1107. Unlike the bits in FIGS. 8 and10, bit 1101 has no protruding shaft on the far end of the bit from thedrive shaft. The bit is designed without a built-in collar or bushing.

Instead, the bit 1101 is designed to spin in bushing 1102. FIG. 12schematically shows three views of the bushing 1102. The bushing 1102 isgenerally arcuate in shape and an arc through the center of the bushing,shown as a dashed line, defines a radius of curvature R₃. The bushing1102 defines a channel 1108 A channel 1108 through the bushing is sizedto receive the drive shaft 1107 of the bit 1101.

FIG. 13 schematically shows a mount 1103 for use with the bit 1101 andbushing 1102. Clockwise from top left, FIG. 13 shows four views of themount 1103: a top view; an angled perspective view showing, top, sideand front; a side view; and a front view. The mount 1103 is to beattached to the bone where the surface will be prepared, and may beaffixed, for example with screws or the like through the anchoring holes1109. The mount defines a bushing slot 1110, sized and shaped to receivethe bushing 1102, having the same arcuate shape and radius of curvatureof the bushing 1102. In use, the bushing 1102 can slide side to side inthe mount 1103. In some embodiments the rotating bit 1101 can slidefront to back through the bushing, and the mount 1103 is deep enough incomparison to the bit 1101 that there is room to move the bit forwardand backward. In other embodiments, the bit 1101 is roughly the samedepth as the depth of the mount 1103 so that the bit only moves side toside. The mount can also define an entry slot 1111 through which the bitcan be introduced into the cutting position inside the mount 1103.

The set-up shown in FIGS. 11-13, like the guide and bit in FIG. 8, canbe used to create a stabilizing surface with curvilinear independentgeometrical aspects, or it can be simplified to keep the independentgeometrical aspects essentially rectangular. As shown, the bushing 1102rides in bushing slot 1110 which is cut in to a curved front wall of themount 1103. The bit 1101 must rotate as the bushing 1102 moves in theslot 1110 in order to keep the bushing 1102 fully seated. In someembodiments, the bushing 1102 may be constrained to so move, for exampleby a mating surface with the mount 1103, e.g., a tongue and groove orsimilar arrangement. Alternatively, if the mount 1103 had a flat front,the bit 1101 would not have to rotate at all to keep the bushing 1102fully seated, and the resulting surface would have rectilinearindependent geometrical aspects.

In any of the preceding examples, or in other embodiments, a kitcomprising a bone-cutting bit and a guide can comprise a kit for cuttinga prepared surface on a bone so as to be complementary to a particularspacer. A kit could include the spacer. A kit could include bushings andor collars necessary to generate a particular radius of curvature.

In any of the preceding examples, or in other embodiments, a kitcomprising a bone-cutting bit and a guide can comprise a kit for cuttinga prepared surface on a bone so as to be complementary to a particularspacer. A kit could include the spacer. A kit could include bushings andor collars necessary to generate a particular radius of curvature.

The joint to be partially replaced can be any synovial joint in thebody. The disclosed devices and methods may be especially beneficial insmall joints such as in the extremities. The joint can be in the foot,for example, the tibiotalar, talonavicular, metatarsal phalangeal,metatarsal tarsal, navicularcuneiform, calcaneal cuboid, subtalar, andinterphalangeal (distal and proximal) joints. The joint can also be inthe hand or wrist, for example, the carpometacarpal joint of the thumb,the scapho-trapezio-trapezoid (STT), the metacarpal-phalangeal joint ofthe thumb, and the proximal interphalangeal joints. Elbow, shoulder andknee joints may also be well suited to the disclosed methods anddevices.

The combination of two or more generating curves with non-constantcurvature can result in a wide variety of different surfaces and shapes,allowing the spacer and complementary surface to mimic a wide variety ofbones and to function in a wide variety of joints. The resultingcomplementary surfaces on the bone and the spacer range from simple tocomplex. In various embodiments, the desired stability can be achievedwith an implant whose stabilizing surface has at least three differentcurvatures distributed across two different axes or geometricallyindependent aspects. It should be understood that in general the goal ofpreparing a bone with such a surface is to allow the prepared surface tomate with a complementary stabilizing surface on a spacer, although onecould prepare a bone with a cut surface for another purpose.

Any of the spacers described herein can be made of a variety ofbiocompatible materials, including pyrolytic carbon. In particular, thearticulating and stabilizing surfaces may be formed entirely ofpyrolytic carbon, as may the bump, if the spacer includes one. Pyrolyticcarbon has been used as an implant material for several decades inartificial heart valves and artificial joint implants. When used withina joint, the material exhibits extreme biocompatibility, surpassingcommon implant metals such as stainless steels, titanium alloys,ceramics, and cobalt chrome alloys. This extreme biocompatibility allowsthe body to act in two advantageous ways. First, pyrolytic carbon cantransmit sliding motion under load to synovial joint surfaces whileallowing the surfaces to remain healthy and functional over prolongedperiods of time. Its biocompatibility against articular cartilageclinically surpasses all presently-known common implant metals. Thisproperty provides prolonged load bearing contact between the jointspacer and the remaining cartilage surface. The second advantage ofpyrolytic carbon is its reaction in the presence of newly formed bonesurfaces, such as are created when the end of a diseased bone isresected. In this case, pyrolytic carbon appears to encourage theformation of a new load transfer surface that responds favorably tosmall induced motions while transmitting joint loads. This propertyleads to the advantageous element of the claimed joint spacer, namelythe absence of the need for rigid fixation between the spacer and thebone.

A spacer useful in hemi-joint replacement for replacing a removedportion of a first bone and articulating with a second bone can have anarticulating surface and a stabilizing surface. The articulating surfacecan be sized and shaped to articulate with an articular surface of thesecond bone. The stabilizing surface can be sized and shaped to conformto a cut surface of the first bone. The spacer can define a first axisand a second axis not parallel to the first axis. The first and secondaxes may or may not be perpendicular. One or both axes can be lines orcurves. In a cross-section of the spacer perpendicular to the firstaxis, the stabilizing surface defines a first curve. In a cross-sectionof the spacer perpendicular to the second axis, the stabilizing surfacedefines a second curve. The first curve can have a first portion with afirst radius of curvature and a second portion with a second radius ofcurvature, where the first and second radii of curvature may be unequal.The second curve can have a third portion with a third radius ofcurvature and a fourth portion with a fourth radius of curvature, wherethe third and fourth radii of curvature may be unequal. Moreover, thesecond and fourth radii of curvature may be unequal as well.

The first and third radii of curvature may be equal to each other, andequal to the radius of curvature of a curved cutting blade of a boneshaving instrument. The second curve can further include a fifth portionwith a fifth radius of curvature. The fifth radius of curvature can beunequal to both the third and fourth radii of curvature. The fifthradius of curvature could be, for example, infinite meaning that thefifth portion is substantially flat.

The stabilizing surface may include a bump, or protrusion, designed tosit inside a cavity defined by the prepared cut surface of the firstbone. The bump would partially, but not entirely, fill the cavity sothat, when the stabilizing surface is fully seated on the cut surface,the bump would protrude into the cavity. The movement of spacer on thecut surface could then be limited by the amount of movement possible bythe bump in the cavity. Such arrangements are described in U.S. Pat. No.8,303,664, which is incorporated herein by reference.

The stabilizing surface may be convex, concave, saddle-shaped, or a morecomplex combination of many radii of curvature. The perimeter of thespacer may be a circle, oval, ellipse, torus, trapezoid, parallelogram,kite, triangle, or any other shape that would help the spacer to mimicthe anatomy of the replaced portion of bone. The exterior of the spacermay consist entirely, or essentially, of the articulating surface andthe stabilizing surface, and no other, or essentially no other,surfaces. The first curve may consist entirely, or essentially, of thefirst and second portions, and may have no other, or essentially noother, portions. The same may be true for the second curve. Either thefirst or second axis, or both, may be curved.

The spacer can include pyrolytic carbon, can consist essentially ofpyrolytic carbon, or can consist entirely of pyrolytic carbon. Inparticular, the articulating surface and/or the stabilizing surface maybe formed of pyrolytic carbon.

Any such spacers may be included in a kit along with a bone-cutting bithaving a cutting portion and a guide sized and shaped to guide the bonecutting bit. The bone-cutting bit and the guide can be sized and shapedsuch that, when the bone-cutting bit is guided by the guide, the cuttingportion sweeps out a surface complimentary to the stabilizing surface ofthe spacer. A profile of the cutting portion can define a curve at leasta portion of which has the first radius of curvature and a profile ofthe guide can define a curve at least a portion of which has the fourthradius of curvature. A profile of the cutting portion can be congruentto the first curve.

A joint that includes a first bone having a first articular surface anda second bone having a second articular surface can be distracted. Thefirst bone can be prepared in at least two steps. First, the first bonemay be prepared by removing the first articular surface, therebycreating a cut surface on the first bone, the cut surface defining afirst axis and a second axis not parallel to the first axis. Second, thefirst bone may be prepared by shaping the cut surface so that: in across-section of the cut surface perpendicular to the first axis, thecut surface defines a first curve including a first portion with a firstradius of curvature, and a second portion with a second radius ofcurvature not equal to the first radius of curvature; in a cross-sectionof the cut surface perpendicular to the second axis, the cut surfacedefines a second curve including a third portion with a third radius ofcurvature, and a fourth portion with a fourth radius of curvature notequal to the third radius of curvature; and the second and fourth radiiof curvature are not equal. A spacer may be placed against the cutsurface of the first bone, the spacer having an articulating surfacesized and shaped to articulate with an articular surface of the secondbone, and a stabilizing surface sized and shaped to be substantiallycomplementary to the cut surface. The joint may be reapproximated suchthat the articulating surface of the spacer articulates with the secondarticular surface of the second bone, and the stabilizing surface of thespacer fully seats against the cut surface of the first bone.

A joint that includes a first bone having a first articular surface anda second bone having a second articular surface can be distracted. Aspacer may be provided, the spacer having an articulating surface sizedand shaped to articulate with an articular surface of the second bone,and a stabilizing surface a predetermined shape. The first bone can beprepared by removing the first articular surface of the first bonethereby creating a cut surface sized and shaped to be substantiallycomplementary to the stabilizing surface. The spacer can be placedagainst the cut surface of the first bone with the stabilizing surfacefacing the complementary cut surface of the first bone. The joint can bereapproximated such that the articulating surface of the spacerarticulates with the second articular surface of the second bone, andthe stabilizing surface of the spacer fully seats against the cutsurface of the first bone. The stabilizing surface can be defined by acombination of at least two surface-generating curves such that twogeometrically independent aspects of the stabilizing surface are definedby non-constant curvature.

A spacer useful in hemi-joint replacement for replacing a removedportion of a first bone and articulating with a second bone can have anarticulating surface and a stabilizing surface. The articulating surfacecan be sized and shaped to articulate with an articular surface of thesecond bone. The stabilizing surface can have a shape defined by acombination of at least two surface-generating curves such that twogeometrically independent aspects of the stabilizing surface are definedby non-constant curvature. The two geometrically independent aspects canbe rectilinear axes, which may or may not be perpendicular to oneanother. The two geometrically independent aspects can also be definedby a curvilinear coordinate system. Curvilinear coordinate systemsinclude, for example polar, bipolar, parabolic, elliptic, hyperbolic,circular cylindrical, parabolic cylindrical, elliptic cylindrical,hyperbolic cylindrical, spherical, oblate spheroidal, prolatespheroidal, and toroidal coordinates. Any system in which geometricallyindependent aspects are defined in space can suffice.

Any such spacers may be included in a kit along with a bone-cutting bithaving a cutting portion and a guide sized and shaped to guide the bonecutting bit. The bone-cutting bit and the guide can be sized and shapedsuch that, when the bone-cutting bit is guided by the guide, the cuttingportion sweeps out a surface complimentary to the stabilizing surface ofthe spacer. A profile of the cutting portion can define one of the atleast two surface-generating curves and a profile of the guide candefine another of the at least two surface-generating curves.

1. A spacer for (a) replacing a removed portion of a first bone, and (b)articulating with a second bone, the spacer comprising: an articulatingsurface sized and shaped to articulate with an articular surface of thesecond bone; and a stabilizing surface sized and shaped to conform to acut surface of the first bone, wherein: the spacer defines first axisand a second axis not parallel to the first axis; in a cross-section ofthe spacer perpendicular to the first axis, the stabilizing surfacedefines a first curve including: a first portion with a first radius ofcurvature; and a second portion with a second radius of curvature notequal to the first radius of curvature; in a cross-section of the spacerperpendicular to the second axis, the stabilizing surface defines asecond curve including: a third portion with a third radius ofcurvature; and a fourth portion with a fourth radius of curvature notequal to the third radius of curvature; and the second and fourth radiiof curvature are not equal.
 2. The spacer of claim 1 wherein the firstand third radii of curvature are equal to each other, and equal to theradius of a curved cutting blade of a bone-shaving instrument.
 3. Thespacer of claim 1 wherein the second curve further includes a fifthportion with a fifth radius of curvature that is (a) not equal to thethird radius of curvature and (b) not equal to the fourth radius ofcurvature.
 4. The spacer of claim 3 wherein the fifth portion issubstantially flat.
 5. The spacer of claim 1 wherein the first axis isperpendicular to the second axis.
 6. The spacer of claim 1 wherein thestabilizing surface is (a) convex, (b) concave, or (c) saddle-shaped. 7.The spacer of claim 1 wherein the perimeter of the spacer issubstantially (a) elliptical, (b) circular, (c) trapezoidal, or (d)toroidal.
 8. The spacer of claim 1 wherein the spacer further comprisesa bump protruding from the stabilizing surface.
 9. The spacer of claim 1wherein the exterior of the spacer consists essentially of thestabilizing surface and the articulating surface.
 10. The spacer ofclaim 1 wherein the first curve consists essentially of the firstportion and the second portion.
 11. The spacer of claim 1 wherein thefirst axis is curved.
 12. The spacer of claim 1 wherein the spacercomprises pyrolytic carbon.
 13. The spacer of claim 12 wherein thearticulating surface and the stabilizing surface are formed entirely ofpyrolytic carbon.
 14. A method comprising: distracting a joint thatcomprises a first bone having a first articular surface and a secondbone having a second articular surface; preparing the first bone by:removing the first articular surface of the first bone thereby creatinga cut surface on the first bone, the cut surface defining a first axisand a second axis not parallel to the first axis; shaping the cutsurface so that: in a cross-section of the cut surface perpendicular tothe first axis, the cut surface defines a first curve including a firstportion with a first radius of curvature, and a second portion with asecond radius of curvature not equal to the first radius of curvature;in a cross-section of the cut surface perpendicular to the second axis,the cut surface defines a second curve including a third portion with athird radius of curvature, and a fourth portion with a fourth radius ofcurvature not equal to the third radius of curvature; and the second andfourth radii of curvature are not equal; placing a spacer against thecut surface of the first bone, the spacer having an articulating surfacesized and shaped to articulate with an articular surface of the secondbone, and a stabilizing surface sized and shaped to be substantiallycomplementary to the cut surface; and reapproximating the joint suchthat the articulating surface of the spacer articulates with the secondarticular surface of the second bone, and the stabilizing surface of thespacer fully seats against the cut surface of the first bone.
 15. Amethod comprising: distracting a joint that comprises a first bonehaving a first articular surface and a second bone having a secondarticular surface; providing a spacer having an articulating surfacesized and shaped to articulate with an articular surface of the secondbone, and a stabilizing surface a predetermined shape; and preparing thefirst bone by removing the first articular surface of the first bonethereby creating a cut surface sized and shaped to be substantiallycomplementary to the stabilizing surface; placing the spacer against thecut surface of the first bone with the stabilizing surface facing thecomplementary cut surface of the first bone; and reapproximating thejoint such that the articulating surface of the spacer articulates withthe second articular surface of the second bone, and the stabilizingsurface of the spacer fully seats against the cut surface of the firstbone; wherein the shape of the stabilizing surface is defined by acombination of at least two surface-generating curves such that twogeometrically independent aspects of the stabilizing surface are definedby non-constant curvature.
 16. A spacer for (a) replacing a removedportion of a first bone, and (b) articulating with a second bone, thespacer comprising: an articulating surface sized and shaped toarticulate with an articular surface of the second bone; and astabilizing surface having a shape defined by a combination of at leasttwo surface-generating curves such that two geometrically independentaspects of the stabilizing surface are defined by non-constantcurvature.
 17. The spacer of claim 16 wherein the two geometricallyindependent aspects are perpendicular rectilinear axes.
 18. The spacerof claim 16 wherein the two geometrically independent aspects aredefined by a curvilinear coordinate system.
 19. A kit comprising: aspacer according to claim 1; a bone-cutting bit having a cuttingportion; and a guide sized and shaped to guide the bone cutting bit;wherein the bone-cutting bit and the guide are sized and shaped suchthat, when the bone-cutting bit is guided by the guide, the cuttingportion sweeps out a surface complimentary to the stabilizing surface ofthe spacer.
 20. The kit of claim 19 wherein: a profile of the cuttingportion defines a curve at least a portion of which has the first radiusof curvature; and a profile of the guide defines a curve at least aportion of which has the fourth radius of curvature.
 21. The kit ofclaim 20 wherein a profile of the cutting portion is congruent to thefirst curve.
 22. A kit comprising a spacer according to claim 16; abone-cutting bit having a cutting portion; and a guide sized and shapedto guide the bone cutting bit; wherein the bone-cutting bit and theguide are sized and shaped such that, when the bone-cutting bit isguided by the guide, the cutting portion sweeps out a surfacecomplimentary to the stabilizing surface of the spacer.
 23. The kit ofclaim 22 wherein a profile of the cutting portion defines one of the atleast two surface-generating curves; and a profile of the guide definesanother of the at least two surface-generating curves.